The present invention is concerned with a brake pressure controller for a hydraulic vehicle brake system. The brake system includes a master cylinder, at least one wheel brake cylinder for actuating a brake and a brake conduit for providing pressure fluid communication between the wheel brake cylinder and the master brake cylinder. A brake pressure control circuit is provided in the brake conduit between the master brake cylinder and the wheel brake cylinder.
The brake system also includes a pump and control valves, controllable through a controller, which connect the wheel brake cylinder to the brake conduit or a return conduit leading to the inlet of the pump. The control valves include at least one valve provided in the brake conduit between the master brake cylinder and the brake pressure control circuit, which at least one valve, in a first operating mode, connects the outlet of the pump, through the brake conduit, to the master brake cylinder, and, in a second operating mode, discontinues the connection between the outlet of the pump and the master brake cylinder and permits a connection between the inlet of the pump and the master brake cylinder or a reservoir.
A brake pressure controller of the above-described type is known from German Patent No. DE-OS 38 31 426. The automatic control system of the brake pressure acting on the wheel brake cylinders responds to the rotating movement of one or several wheels of an automotive vehicle monitored by a monitoring system. The control system prevents the wheels from locking during a braking operation through brake slip control, and prevents the driven wheels from racing, during start-up, through traction slip control.
The first mode of operation is provided for brake slip control while the second mode of operation is provided for traction slip control. In known brake pressure control systems, the brake pressure cylinder, in the first mode of operation during a pressure reducing step for lowering the brake pressure, is connected only to a low-pressure accumulator and to the inlet of the pump. If the low pressure accumulator is empty, there is a danger that the pressure in the wheel brake cylinder may be lowered, through the intake force of the pump, to such an extent that a retraction of the brake piston and an ingress of air are likely to occur, which might affect the braking operation and the brake pressure control.